The present invention relates to an ultrasound transducer device with a one-dimensional or two-dimensional array of transducer elements.
An ultrasound transducer device is disclosed in the publication "Ultrasonics", March 1981, pages 81 to 86, and in Patent Document No. GB-A-2,052,918.
In the field of ultrasound imaging technology, a body to be examined, especially a human body, is exposed to ultrasonic waves using ultrasound pulses. An ultrasound image is built up from ultrasound echo pulses reflected at the border surfaces of this body, using an electronic signal processing unit, with the echo amplitudes and the echo run times serving as data. For example, such a method is disclosed in "Proc. IEEE", Vol. 67, No. 4, April 1979, pages 620 to 641. To send and receive the ultrasound pulses according to such a pulse-echo method, piezoelectric transducer elements are preferably used. These transducer elements can be arranged in a linear (one-dimensional) row or chain (a so-called linear array) and are controlled by an electronic control unit, separately or in groups, to achieve a directing effect. The control of the sonic beam takes place by time-delayed transmission of the individual elements in the transmission case, where the desired beam direction results from superimposition of the waves proceeding from the elements, pursuant to Huygens' principle. In the reception case, the desired angle-dependent sensitivity is also achieved by time-dependent or phase-dependent superimposition of the time signal progressions recorded by the individual elements. Corresponding arrays of ultrasound transducer elements are therefore also referred to as "phased arrays."
Using such phase-delayed controlled linear arrays, ultrasound beams that can be pivoted and focused in a plane formed by the normal line on the array surface and the longitudinal direction of the array can be sent and received. The pivot angle measured relative to the normal line for the ultrasound beam increases as the distance between the transducer elements decreases. This distance is generally selected to be approximately equal to half the wavelength .lambda. of the ultrasound, in order to suppress additional diffraction patterns in this way, and amounts to about 0.2 mm, for example, at a mean frequency of 3.5 MHz. On the other hand, a certain minimum length of the linear array is necessary, in order to achieve sufficient sonic amplitude and exact focusing of the beam. From these two requirements with regard to the reciprocal distance of the transducer elements and the minimum length of the array, a minimum number of transducer elements for the array is derived, which minimum number of elements is typically 64 or higher.
For any desired beam direction of an ultrasound beam in all three dimensions, which are a prerequisite for imaging moving bodies such as blood flow in the heart or arteries, the one-dimensional, linear array must be expanded to form a two-dimensional matrix (a so-called 2D array arrangement). Such a two-dimensional matrix array is disclosed, for example, in "Ultrasonics Imaging", Vol. 14, 1992, pages 213 to 233. Such matrix arrays for a three-dimensional beam control (e.g.,see "IEEE Trans. Ultrason., Ferroel., Frequ. Contr.", Vol. 38, No. 4, July 1991, pages 320 to 333) must fulfill certain conditions with regard to their lateral and axial resolution capacity, in order to be suitable for diagnostic purposes. While the axial resolution capacity is primarily determined by the frequency given off and the band width of the necessary electronics, the lateral resolution capacity is established by the frequency and the effective aperture of the array. For corresponding commercial ultrasound transducer devices, the following values are typical:
Mean frequencies: 3.5 to 10 MHz, effective aperture: 19 to 10 mm, band width: .gtoreq.50% (6 dB) with reference to the mean frequency.
If the resolution capacity which can be achieved with this is also used as a basis for 2D matrix arrays, then individual element distances of 0.2 to 0.075 mm and matrices of at least 64.times.64, particularly 100.times.100 elements, are obtained on the basis of the .lambda./2 conditions mentioned. These 10,000 elements must be housed on an area of 50 mm.times.50 mm, for example, with their sound-producing surfaces being approximately 0.2 mm.times.0.2 mm each. The thickness of the individual elements is 0.35 mm (for 3.5 MHz) to less than 0.2 mm (for 10 MHz), depending on the frequency and the piezo ceramics used. In addition, an over-response attenuation should reach about 30 dB between the individual elements and the electrical transmission and reception channels assigned to them, both acoustically and electronically. This requires significant efforts with regard to a suitable acoustic attenuation element, a so-called backing, and also with regard to insulation of the individual electrical supply lines. There can be no corresponding number of electronic transmission and reception channels to stand against the large number of individual elements. Therefore, effective utilization of the maximum possible number of channels is required, and can be achieved by means of multiplexer circuits. For both modes of operation (i.e., transmission and reception), electronic circuit parts are therefore required in the vicinity of the piezoelectric transducer elements. It is most practical to structure these circuit parts as integrated circuits, which are to be arranged on the acoustical shadow side of the transducer elements. In this connection, a high packing density of the elements leads to problems with regard to the contacting and structural technology, as well as with regard to the power dissipation of the electronic circuit parts.
From the "Ultrasonic" publication mentioned above and from Patent Document No. GB-A-2,052,918, an ultrasound transducer device with a plurality of ultrasound transducer elements is known, with the transducer elements being arranged in a two-dimensional matrix. Here, the transducer device is composed of several plate-shaped components. Each of these components contains a plate-shaped acoustic attenuation element, at the narrow side of which a one-dimensional row (line) of transducer elements is arranged. These transducer elements are formed by a corresponding subdivision of a strip-shaped piezoelectric element. For each of the oscillation elements of an element formed in this way, the two required electrodes are affixed at opposite side surfaces, so that the electrodes are aligned parallel to the sonic beam direction. The electrical connection leads connected to this run via the corresponding two broad sides of the attenuation element and lead to an electronic circuit pan affixed at the attenuation element. In the known embodiment, the circuit pan of each component of the transducer device is therefore composed of two circuit subunits located on the opposite broad sides of their attenuation element. On the basis of this bilateral arrangement of circuit subunits on each attenuation element, the packing density of the entire transducer device consisting of the individual components is therefore correspondingly limited. This restriction is also due to the fact that the electronic circuit parts on the attenuation element are not cooled and thus only a limited power dissipation of the electronics can be permitted. Furthermore, the structure provided for the known transducer device cannot be easily provided for higher frequencies and thus smaller dimensions of the elements, due to a minimum thickness of the silicon of over 0.1 mm. Furthermore, since the elements which emit sonic waves are each connected with the attenuation element only over part of the surface, at their lower part, this impairs their sonic wave emission behavior in disadvantageous manner.